The invention relates to a field mounted measurement transmitter powered by a process control loop which transmits a process variable over the control loop. More specifically, the invention relates to a power supply for the field mounted transmitter.
Process control systems are used in industry to monitor and control process. A field mounted transmitter monitors a variable of the process, such as temperature, flow rate, or pressure. The transmitter is coupled to a two-wire process control loop which carries a loop current, I. Loop current I powers the transmitter. The transmitter sends the process variable over the control loop to a central controller which monitors the entire process. The central controller uses the process variable received from the transmitter to control the process.
The power which is available for use by the transmitter is limited by the control loop. Typical control loops include a 4-20 mA standard in which a 4 mA signal represents a zero and a 20 mA signal represents a full scale reading. The transmitter transmits the process variable by controlling current I. Digital information may also be transmitter on the same loop. Another standard is the field bus standard in which multiple transmitters can couple to the same control loop.
It is desirable to provide the field mounted transmitters with a high level of computational ability. The advantages of such ability are numerous and include error correction, data logging, and diagnostic functions. However, power limitations put a constraint on the processing capabilities of the transmitter. For example, if a microprocessor is performing the calculations its clock speed must be slowed appropriately to limit power draw.
Therefore, power constraints are a limiting factor in the capabilities of field mounted transmitters.